At least 64 people were killed in a police operation against the Red Command gang in Rio de Janeiro's north zone - the biggest and most lethal operation in the Brazilian state's history.
Four security agents were included in the fatalities during Operation Containment in the communities of Alemão and Penha, local authorities said. In excess of 80 individuals were detained.
Intense confrontations erupted when around 2,500 law enforcement officers moved in on Tuesday early hours. The criminal organization countered with gunfire, burning barricades and dropping bombs from drones, authorities stated.
The United Nations human rights body said it was "horrified" by the police operation, calling for "prompt and effective investigations".
The informal settlements close to the city's international airport is considered to be the central command of one of Brazil's most powerful organised crime groups.
Local authorities stated no less than 50 of the deceased were "indicated by police as suspected of being criminals".
Many residents were injured, including innocent bystanders trapped in shooting.
Locals have compared the hillside area - inhabited by approximately 300,000 people - to a "war zone" since the operation began.
Transport routes stay blocked in the entire neighborhood.
The criminal network has been increasing their territory across the Rio state, growing illegal substance trade paths and dominance over the slum communities.
Rio's police said over 200kg (441lb) of drugs were captured in the operation, together with dozens of firearms.
The coordinated mission by different law enforcement agencies follows a year-long investigation.
Numerous alleged criminals are thought to be affiliates of a group from the Pará state who are presently concealed in Rio de Janeiro.
Extensive security missions are regular in the country ahead of important worldwide conferences in the country.
The UN climate summit the climate conference is set to occur in the northern city of Belem in November.
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